1. Field of the Invention
The present invention relates to an analog signal detecting circuit in which reference potentials at signal input/output terminals are different from each other, and an AC side current detector of a semiconductor power conversion device using the same, and particularly, to an analog signal detecting circuit using a HVIC (High Voltage Integrated Circuit) characterized by including a non-optical semiconductor element, and a dead time fixing method of input/output signals.
2. Description of the Related Art
A conventional analog signal detecting circuit will be described with reference to the drawings. FIG. 6 is a view showing a structure of a conventional analog signal detecting circuit. FIG. 6 shows an example of the conventionally used analog signal detecting circuit in which reference potentials at signal input/output terminals are different from each other.
In FIG. 6, reference numeral 1 designates an analog signal detecting circuit; 2, a signal input terminal; 3, a signal output terminal; 4, a reference potential (GNDin) at the signal input terminal 2; 5, a reference potential (GNDout) at the signal output terminal 3; 6, a power supply (Vrefin) at the signal input terminal 2; 7, a power supply (Vrefout) at the signal output terminal 3; 8, a V/F converter; 9, a F/V converter; 10, a photocoupler; 11, a power supply system with the Vrefin and GNDin; and 12, a power supply system with the Vrefout and GNDout.
Next, the operation of the foregoing conventional analog signal detecting circuit will be described with reference to the drawing.
An analog signal is inputted to the V/F converter 8 through the signal input terminal 2. This V/F converter 8 is operated through the same power supply system 11 as the signal input terminal 2. The V/F converter 8 outputs a digital signal having a frequency uniquely corresponding to the signal level of the analog input signal.
The digital signal outputted from the V/F converter 8 is electrically insulated by the photocoupler 10 and is connected to the F/V converter 9 operating through the same power supply system 12 as the signal output terminal 3. This F/V converter 9 outputs an analog signal of a level uniquely corresponding to the frequency of the inputted digital signal.
As described above, in this prior art, an analog signal is converted into a digital signal having a frequency uniquely corresponding to the level of the input analog signal, is electrically insulated by the photocoupler 10, and is converted into an analog signal of a level uniquely corresponding to the frequency of the digital signal. In this way, even in the case where reference potentials at the input/output terminals are different from each other, the analog signal can be detected.
Subsequently, a conventional AC side current detector of a semiconductor power conversion device will be described with reference to the drawings. FIG. 7 is a view showing a structure of a conventional AC side current detector of a semiconductor power conversion device using the analog signal detecting circuit of FIG. 6.
In FIG. 7, reference character 1a designates a U-phase analog signal detecting circuit; 1b, a V-phase analog signal detecting circuit; 1c, a W-phase analog signal detecting circuit; 2a, a U-phase signal input terminal; 2b, a V-phase signal input terminal; 2c, a W-phase signal input terminal; 3a, a U-phase signal output terminal; 3b, a V-phase signal output terminal; 3c, a W-phase signal output terminal; 4a, a reference potential (GNDinu) at the U-phase signal input terminal 2a; 4b, a reference potential (GNDinv) at the V-phase signal input terminal 2b; 4c, a reference potential (GNDinw) at the W-phase signal input terminal 2c; 6a, a power supply (Vrefinu) at the U-phase input terminal 2a; 6b, a power supply (Vref inv) at the V-phase signal input terminal 2b; and 6c, a power supply (Vrefinw) at the W-phase signal input terminal 2c.
Beside, in the drawing, reference character 8a designates a U-phase V/F converter; 8b, a V-phase V/F converter; 8c, a W-phase V/F converter; 9a, a U-phase F/V converter; 9b, a V-phase F/V converter; 9c, aW-phaseF/Vconverter; 10a, aU-phasephotocoupler; 10b, a V-phase photocoupler; 10c, a W-phase photocoupler; 11a, a power supply system with the Vrefinu and GNDinu; 11b, a power supply system with the Vrefinv and GNDinv; and 11c, a power supply system with the Vrefinw and GNDinw.
Beside, in the drawing, reference character 13a designates U-phase current voltage conversion means; 13b, V-phase current voltage conversion means; 13c, W-phase current voltage conversion means; 14, a load; and 15, a semiconductor power conversion device.
Furthermore, in the drawing, reference character 16a designates a U-phase P-side switching element; 16b, a V-phase P-side switching element; 16c, a W-phase P-side switching element; 16d, a U-phase N-side switching element; 16e, a V-phase N-side switching element; 16f, a W-phase N-side switching element; 17a, a U-phase P-side flywheel diode; 17b, a V-phase P-side flywheel diode; 17c, a W-phase P-side flywheel diode; 17d, a U-phase N-side flywheel diode; 17e, a V-phase N-side flywheel diode; 17f, a W-phase Nside flywheel diode; and 18, an AC side current detector.
In FIG. 7, the semiconductor power conversion device 15 converts DC electric power from a not-shown power supply device into AC electric power, and supplies it to the load 14. For example, the semiconductor power conversion device 15 is an inverter, and the load 14 is a three-phase AC electric motor.
At this time, the conversion from the DC electric power to the AC electric power is carried out by switching the switching elements constituting a power element of the semiconductor power conversion device 15. Note that the power element is constituted by the switching elements 16a to 16f and the flywheel diodes 17a to 17f. In general, in the semiconductor power conversion device 15, switching control of the switching elements and protection of the power element are carried out by detecting the U-phase, V-phase, and W-phase currents.
In the AC side current detector 18, first, the U-phase, V-phase, and W-phase currents are converted into voltage signals by the current voltage conversion means 13a to 13c. For example, the current voltage conversion means 13a to 13c are shunt resistors. The voltage signals outputted from the respective U-phase, V-phase, and W-phase current voltage conversion means 13a to 13c are respectively operated on the electrically insulated power supply systems 11a to 11c, and are respectively converted into digital signals uniquely corresponding to the voltage signal levels by the V/F converters 8a to 8c operating on the respective power supply systems 11a to 11c.
Any of the U-phase, V-phase, and W-phase signal output terminals 3a to 3c of the AC side current detector 18 operates on the same power supply system 12. The digital signals outputted from the V/F converters 8a to 8c are electrically insulated by the photocouplers 10a to 10c, and are inputted to the F/V converters 9a to 9c. The F/V converters 9a to 9c convert the digital signals outputted from the photocouplers 10a to 10c into voltage signals of levels respectively uniquely corresponding to the frequencies of the digital signals.
As described above, in the conventional AC side current detector of the semiconductor power conversion device, AC side current detection of the semiconductor power conversion device is made possible by using the above conventional analog signal detecting circuit.
In the foregoing conventional analog signal detecting circuit, even in the case where potential references are different from each other at the input/output terminals, detection of the analog signal becomes possible. However, since the photocoupler 10 is used for electrical insulation of the digital signal, it is impossible to provide an analog signal detecting circuit having high reliability and long lifetime under a bad environment.
Besides, since the input analog signal is temporarily converted into the digital signal having a frequency uniquely corresponding to the analog signal level by using the V/F converter 8, there has been a problem that a dead time of the analog signal detecting circuit is changed in accordance with the analog signal level.
In the foregoing conventional AC side current detector of the semiconductor power conversion device, it is possible to detect an AC side current of the semiconductor power conversion device 15. However, the dead time of the analog signal detecting circuits 1a to 1c used in the AC side current detector 18 of the semiconductor power conversion device 15 is changed similarly.
Since the V/F converter 8a to 8c of the respective phases independently convert signals into digital signals each having a frequency uniquely corresponding to the analog signal level, with respect to the AC side current detection values at the signal output terminals 3a to 3c of the respective phases of the AC side current detector 18, concurrency can not be held at the respective phases.
In the case where protection of the power element constituting the semiconductor power conversion device 15 is made by using the thus detected AC side current, an operation delay follows when the frequency of the digital signal is low, and the protecting operation is not carried out well. Also in the case where it is used as a feedback value of current control, when the frequency of the digital signal is low, the response frequency of the current control system can not be set high.
Moreover, since the concurrency is not held, an abnormal component is superimposed on a composite vector of the AC side currents, high performance load control (for example, vector control of an AC electric motor or power factor control of a PWM converter) can not be realized.